In heavy duty radial tires such as tires for trucks and buses, tires for construction vehicles and the like, a main belt layer is generally constructed by combining a large slant belt layer(s), in which cords of a rubberized cord layer are arranged side by side at a large inclination angle with respect to an equatorial plane of the tire, for example at an inclination angle of 20–70° and usually embedded at equal intervals, with a small slant belt layer(s), in which cords of a rubberized cord layer are arranged side by side at a small inclination angle with respect to the equatorial plane of the tire, for example at an inclination angle of 5–15° and usually embedded at equal intervals.
That is, a rigidity to deformation along a face of the belt (hereinafter referred to as in-plane bending rigidity) is ensured by the large slant belt layer. On the other hand, the small slant belt layer bears tension in a circumferential direction of the tread owing to the arrangement of cords having a small inclination angle and controls a growth of a tread size to prevent a change of a crown shape during the running.
Further, the main belt layer is usually comprised of two small slant belt layers and one or two large slant belt layers outward from a side of the carcass in a radial direction or 3 to 4 layers in total. In this case, it is effective that the small slant belt layers are arranged in a direction of crossing cords with each other between the adjoining layers for bearing the tension in the circumferential direction of the tread, and also that when the large slant belt layer located outside the small slant belt layer is comprised of two layers, the cords of these layers are crossed with each other between the adjoining layers for enhancing the in-plane bending rigidity, and that the cords are crossed with each other between the adjoining small slant belt layer and large slant belt layer for preventing strain concentration in the small slant belt layer nearest to the carcass.
In the main belt layer, therefore, it is advantageous to arrange the cords of all layers so as to cross them with each other between the adjoining layers in view of the control of size growth in the tread, the improvement of in plane bending rigidity and the dispersion of strain at belt end.
When the tire is run under loading, the occurrence of shear strain can not be avoided at an end zone of the belt. Especially, as the cord inclination angle in the belt becomes smaller and as the width of the belt becomes wider, the shear strain generated is large, so that cracks are caused at the belt end positioning rubber and cords ends not covered therewith and grow to separation failure between the layers and hence bring about the breakage of the tire.
Here, the large slant belt layer is large in the cord inclination angle and small in the shear strain generated, so that it is possible to widen the belt width and the securement of in-plane bending rigidity as its main object is easy.
On the other hand, since a main object of the small slant belt layer is to realize the control of the tread size growth, it is indispensable that the cord inclination angle is made small and a minimum belt width is ensured, but it is obliged to narrow the belt width or to set the cord inclination angle to a large value for decreasing the shear strain during the running of the tire under loading. In any case, however, the control of the tread size growth becomes insufficient and hence the size growing amount increases during the running and the resistance to cut separation, heat resistance and wear resistance are deteriorated. That is, in case of the small slant belt layer having a small cord inclination angle, it is advantageous to widen the width and make the cord inclination angle smaller for the control of the tread size growth, but the realization of them is obstructed by the occurrence of separation failure starting from the belt end as mentioned above.